The present invention relates generally to innovations and improvements in the non-destructive quantitative analysis of material composition by measuring the change in energy spectrum of neutron radiation that has been transmitted through, or backscattered from, a material. More particularly, the present invention relates to a method for quantifying the amount of hydrogen or hydrogen-bearing constituent in a material using a source of radiation comprising neutrons or a combination of neutrons and gamma rays. xe2x80x9cGamma raysxe2x80x9d used hereinafter include photons produced by changes in electron energy states of an atom (i.e., X-rays) in addition to photons produced by changes in energy states in the nucleus.
There are numerous agricultural, commercial, industrial, geological and security applications requiring the non-destructive determination of the amount of a hydrogen-bearing constituent in a material. For example, there is a need to measure water or fat content in agricultural products (e.g., grain, soybeans, cottonseeds, meat, milk), measure steam quality, measure hydrogen content in fuels, and detect explosives.
Current non-destructive methods for determining composition using penetrating radiation include gamma ray absorptiometry and prompt or delayed gamma neutron activation analysis (PGNAA). Gamma-ray absorptiometry is exemplified by U.S. Pat. Nos. 2,992,332 and 4,168,431 and illustrated in a simplified manner in FIG. 1. The material 100 to be analyzed is exposed to an incident beam of radiation 110 comprising gamma rays from a radiation source 120 (hereinafter referred to simply as xe2x80x9csourcexe2x80x9d) whereby the reduced intensity of the transmitted beam 130 thereof is measured by a detector 140, such as an ionization chamber.
In this method, the gamma rays give up some or all of their energy within the material 100 in three principal ways which are characterized as: the photoelectric effect, the Compton effect, and pair production effect. The photoelectric effect occurs when a gamma ray ( less than xcx9c0.1 MeV) strikes an electron in one of the orbits of an atom, dislodging it therefrom. The gamma ray gives up its entire energy to raising the kinetic energy of the electron (true absorption) equal to the gamma ray energy minus the binding energy of the electron. The Compton effect occurs when a gamma ray ( greater than xcx9c0.1 MeV) also strikes an electron in one of the orbits, dislodging the electron from its orbit, but only a part of the gamma ray energy is used up, and the gamma ray itself is deviated from its path by the collision (scattering absorption). In the pair production effect, the gamma ray ( greater than xcx9c1 MeV) is annihilated in the vicinity of the nucleus of the absorbing atom with the subsequent production of an electron and positron pair.
Another absorptiometric method for determining composition is exemplified by U.S. Pat. No. 5,479,023 which discloses a method of transmitting an incident beam of radiation 110 comprising different species (gamma rays and neutrons) from a source 120 through a material 100. A similar setup to that shown in FIG. 1 is used in this method except that the detector 140 also comprises a thermal neutron detector. In such absorptiometric methods, the transmitted radiation intensity measurements are typically analyzed with reference to known characteristic gamma ray and neutron attenuation coefficients and density properties.
It is important to note that measuring certain characteristics of the transmitted neutrons can provide different compositional information about the material 100 than that gained by measuring the gamma rays alone. This is based on the fact that neutrons interact with the constituents of the material 100 in a different manner than gamma rays. That is, the neutrons give up some or all of their energy in the material 100 by: elastic collision, inelastic collision, and radiative capture. Elastic collision occurs when the neutron shares its kinetic energy with a nucleus without exciting the nucleus. This is the primary mode of energy loss for neutrons as they are slowed or xe2x80x9cmoderatedxe2x80x9d to thermal energies by interaction with light nuclei (e.g., graphite or hydrogenous xe2x80x9cmoderatorsxe2x80x9d such as water and polymers). Inelastic collision usually occurs with fast neutrons whereby the nucleus becomes excited upon collision, emits a gamma ray, and shares the remainder of the available kinetic energy with the scattered neutron. Radiative capture occurs when a neutron is absorbed to produce an excited compound nucleus which attains stability by emission of a gamma ray. Radiative capture is the basis of PGNAA whereby the gamma ray emission spectrum is analyzed to determine composition.
Another neutronic method for determining composition, disclosed in U.S. Pat. No. 5,327,773 and schematically illustrated in FIG. 2, utilizes a backscattering and thermalization technique. In this method, well known to practitioners of the geologic sciences, the density of steam (i.e., the material 100) in a conduit is measured by differentially measuring the thermal and epicadmium backscattered neutron radiation 200 generated by the incident beam of radiation 110 (from the source 120) interacting with hydrogen nuclei in the material 100. A first thermal neutron detector 210 and a second thermal neutron detector 220 (typically shielded with cadmium) are used in such a technique. A disadvantage of this method, however, is the length of the counting times associated with the inefficient detection of epicadmium neutrons.
In all of the aforementioned neutronic methods, however, the analyses do not utilize the great wealth of information contained in the changes in the multiple levels of energy in the neutron energy spectrum between the incident and transmitted, or backscattered, radiation resulting from the excellent neutron moderating property of hydrogen. Accordingly, there is an opportunity with the present invention to replace or improve current hydrogen analysis techniques by a more efficient neutron spectroscopic technique.
The present invention is a non-destructive method for quantifying the amount of hydrogen or a hydrogen-bearing constituent in a material. The method is based on the principle of moderating neutron spectroscopy which is particularly suited for analyzing materials containing the high scattering/capture cross-section element of hydrogen. The method comprises the steps of exposing a neutron moderator to a beam of radiation comprising neutrons and measuring thermal neutron intensities at a plurality of locations in the moderator. These measured intensities reflect the energy distribution of the beam of radiation incident upon the moderator. Thus, by measuring these intensities with a material present, and comparing these intensities to a model, to those of a composition standard(s), or combinations thereof, the hydrogen content of the material is quantified. Furthermore, the hydrogen-bearing constituent of the material is quantified by knowing or estimating the chemical or molecular structure of the material.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.